The present invention relates in a general aspect to methods of decreasing the density of thermoplastic resin foam materials by a process of pressurization of already foamed materials followed by secondary expansion thereof. In a more particular aspect it relates to a method of pressurization and storage of such foam materials prior to secondary expansion.
It is known to produce thermoplastic resin foam materials having substantially closed-cell structures by intimately incorporating within the resin material a volatile organic liquid which vaporizes upon heating to form a gas (i.e., a so-called "physical blowing agent") or a solid substance which decomposes upon heating to form a gas (i.e., a so-called "chemical blowing agent"), and thereafter heating the composition at elevated temperatures whereby the gases created expand the thermoplastic material to form a cellular mass.
It is also known that thermoplastic resin materials, such as polystyrene, which have been foamed by the action of a volatile organic liquid producing a primary foaming gas may thereafter be induced to further expand by exposing the foamed material to another gas, such as steam or air, which has a permeability rate greater than the permeability rate of the primary foaming gas through the cell walls of the foamed mass, and subsequently reheating the foamed material to a heat softening temperature. At the heat softening temperature, the combined effect of the primary gas and the secondary gas, which had entered the cells of the foamed material primarily by osmotic pressure, causes further expansion of the initially foamed material, producing a lower density foam product.
It is further known that thermoplastic resin materials, such as polyethylene, which have been foamed by the gas emitted upon decomposition of a solid substance may thereafter be induced to further expand by heating the foamed material to a temperature near the melting point of the resin while subjecting it to a secondary gas at superatmospheric pressure, and subsequently reheating the foamed material to a heat softening temperature at a lower pressure, e.g., atmospheric pressure, thereby expanding the gas within the foam cells. The combined expansion of the primary gas and the secondary gas, which had entered the cells of the foamed material primarily because of the internal/external pressure differential, produces a lower density foam product.
In U.S. Pat. No. 3,953,558 to Hatano et al., there is disclosed a method of molding foamed synthetic resin pellets of polyolefin. In that process, foamed pellets are first prepared having a plurality of closed cells whose volume is greater than 65% of the total volume of all cells contained in the foamed pellets. Thereafter, the pellets are heated to a temperature up to the melting point of the polyolefin resin, and concurrently subjected to a gas at superatmospheric pressure. After the internal foam cell pressure has increased to an amount greater than 1.18 atmospheres, the external pressure is reduced. The pellets are immediately transferred to a mold and heated to a temperature which expands the entrained gas, thereby increasing pellet size and causing the pellets to adhere to each other, thus forming an article in the shape of the mold.
Unfortunately, however, the above-noted process requires that the pressurized foam pellets be transferred to a mold immediately after they are removed from the pressurizing vessel, therefore prohibiting transportation and/or storage of the pressurized foam pellets. Accordingly, it would be desirable to provide an improved method for pressurizing a pre-foamed thermoplastic resin material and retaining that pressure for prolonged storage periods at atmospheric pressure prior to secondary expansion.